The long-term objective of this proposal is to gain insight into mechanisms of innate immunity to infectious agents in humans, with particular emphasis on the role of Toll-like receptors (TLRs) in cutaneous host defense. We recently reported a role for TLRs in the immune response to microbial pathogens, leading to the induction of IL-12, a powerful signal for the generation of Th1 cytokine responses, and inducible nitric oxide synthase (NOS), a direct microbicidal pathway. We propose experiments to investigate the role of TLRs in skin and the innate response to microbial infection. First, we hypothesize that different TLRs have distinct roles in host immunity. We will determine the role of specific TLRs in the cutaneous immune response by comparing the distribution of TLRs in skin lesions, analyzing the role of distinct TLRs in monocyte and keratinocyte responses to microbial ligands and determining the mechanism by which TLR activation leads to killing of intracellular bacteria. Second we hypothesize that distinct TLRs or TLR signaling pathways lead to immunostimulatory vs. down-modulatory events. We propose to determine the structural characteristics of TLRs that lead to distinct signaling specificities and the signal transduction pathway leading to induction of IL-12 and iNOS vs the induction of IL-10 and macrophage apoptosis. Third, we hypothesize that the expression of TLR on T cells allows these cells of the adaptive immune system to participate in the innate response. We propose to measure the T-cell receptor repertoire of TLR+ T cells and the response of TLR+ T cells to microbial ligands including the cytokine patterns induced and signal transduction pathways activated. We believe that the insights obtained form the study of TLRs in skin and their response to microbial lipoproteins will provide new knowledge about the innate immune response and suggest new avenues of immunologic intervention in human disease.